This invention relates to a hydraulic drive system for a work machine such as a hydraulic excavator, and especially to a hydraulic drive system for a work machine, said hydraulic drive system being provided with a flow-combining valve for combining flows of pressure fluid from two hydraulic pumps and being adapted to perform overall power control such that a total value of all torques including input torques to the two hydraulic pumps does not exceed an output torque from an engine.
FIG. 5 is a hydraulic circuit diagram showing the construction of a conventional hydraulic drive system for a work machine.
The conventional technique illustrated in FIG. 5 is applied, for example, to a hydraulic excavator, and is provided with an engine 30 and a first and second hydraulic pumps 15,18 both of which are of the variable displacement type and are driven by the engine 30. To the first hydraulic pump 15, a first group of directional control valves consisting of plural center-bypassed directional control valves is connected. To the second hydraulic pump 18, a second group of directional control valves consisting of plural center-bypassed directional control valves is connected likewise. In the second group of directional control valves, a flow-combining directional control valve 4 for changing over and controlling a combined-flow-driven actuator 20 is included. To a directional control valve 1 positioned most downstream of the first group of directional control valves connected to the above-mentioned first hydraulic pump 15, a flow-combining valve 2 is connected via a center bypass passage 3 such that pressure fluid from the first hydraulic pump 15 can be supplied, in combination with pressure fluid from the second hydraulic pump 18, to the aforementioned flow-combining directional control valve 4. The flow-combining valve 2 and a supply port of the flow-combining directional control valve 4 are connected to each other by a flow-combining circuit 5.
The aforementioned flow-combining valve 2 is arranged such that, depending on the magnitude of a pilot pressure in a pilot line 7 through which the pilot pressure is guided to change over the flow-combining directional control valve 4, the flow-combining valve 2 is changed over from an open position at which the center bypass passage 3 and a reservoir 17 are communicated with each other to a closed position at which the center bypass passage 3 and the reservoir 17 are cut off from each other or conversely, from the closed position to the open position.
An attachment which is driven by the aforementioned combined-flow-driven actuator 20 comprises a predetermined attachment mounted on a free end of an arm of the hydraulic excavator, for example, a breaker. On the free end of the arm, a bucket is generally mounted. By removing the bucket, this breaker is mounted instead.
FIG. 5 also illustrates a parallel line 21 via which the individual directional control valves included in the second group of directional control valves are connected parallel to the second hydraulic pump 18, a reservoir passage 19 communicating the center bypass passage of the second group of directional control valves and the reservoir 17 with each other, a check valve 22 for preventing pressure oil in the flow-combining line 5 from flowing toward the parallel line 21, and a check valve 6 for preventing the pressure fluid in the flow-combining line 5 from flowing toward the center bypass passage 3.
According to the conventional technique constructed as described above, when any one of the individual directional control valves is changed over except for a change-over operation that the flow-combining directional control valve 4 is changed over to the right position of FIG. 5, no pilot pressure is developed in the pilot line 7, and the flow-combining valve 2 is thus held in the open position by the force of a spring. Namely, the center bypass passage 3 is maintained in communication with the reservoir 17. In this state, change-over of one or more of the directional control valves included in the first group of directional control valves makes it possible to supply the pressure fluid from the first hydraulic pump 15 to the corresponding directional control valve(s) only, and change-over of one or more of the directional control valves included in the second group of directional control valves makes it possible to supply the pressure fluid from the second hydraulic pump 18 to the corresponding directional control valve(s) only.
When the pilot pressure is guided into the pilot line 7 upon driving the combined-flow-driven actuator 20, the flow-combining directional control valve 4 is changed over to the right position of FIG. 5 and at the same time, the flow-combining valve 2 is changed over to the closed position against the force of the spring. As a consequence, the center bypass passage 3 and the reservoir 17 are cut off from each other.
Accordingly, the pressure fluid from the first hydraulic pump 15 is supplied, in combination with the pressure fluid from the second hydraulic pump 18, to the supply port of the flow-combining directional control valve 4 via the center bypass passage 3, the flow-combining circuit 5 and the check valve 6. The combined pressure fluid of the pressure fluid from the first hydraulic pump 15 and the pressure fluid from the second hydraulic pump 18 is supplied from the flow-combining directional control valve 4 to the combined-flow-driven actuator 20. The combined-flow-driven actuator 20 is, therefore, actuated to drive the unillustrated breaker so that breaking work or the like of rocks is performed.
To perform combined operation of the breaker and an unillustrated arm and/or boom or combined operation of the breaker and running and/or revolving, the corresponding one or more of the directional control valves included in the first group of directional control valves, for example, may also be changed over at the same time. In this case, the pressure fluid from the first hydraulic pump 15 is supplied to the corresponding one or more directional control valves. At this time, the center bypass passage(s) of the corresponding one or more directional control valves, in many instances, is(are) not completely closed in actual work, so that there is also a tendency that a portion of the pressure fluid from the first hydraulic pump 15 is also supplied to the flow-combining line 5. In other words, the combined-flow-driven actuator 20 tends to be brought into such a situation that it is driven by the portion of the pressure fluid from the first hydraulic pump 15 and the pressure fluid from the second hydraulic pump 18.
While these operations are carried out, overall power control is performed such that a total value of input torques to the first hydraulic pump 15 and the second hydraulic pump 18 does not exceed an output torque from the engine 30 to avoid stalling.
In the above-described conventional technique, a load pressure on the combined-flow-driven actuator 20 may become high for a certain reason in the course of work that the unillustrated breaker is driven by a combined flow of the pressure fluid from the first hydraulic pump 15 and that from the second hydraulic pump 18. Corresponding to the load pressure, a delivery pressure on the side of the second hydraulic pump 18 then becomes high, and a delivery pressure on the side of the first hydraulic pump 15 also becomes high. As a result, a total value of an input torque to the first hydraulic pump 15 and an input torque to the second hydraulic pump 18 becomes large, and an output torque from the engine 30 also increases.
When the load pressure on the combined-flow-driven actuator 20 becomes high as mentioned above, there is a situation that force is required more than speed. Combining the pressure fluid from the first hydraulic pump 15 with the pressure fluid from the second hydraulic pump 18 in such a situation leads to an increase in the output torque from the engine 30 as mentioned above. As a consequence, the fuel consumption increases, developing a problem in economy.
For example, in the course of combined operation of another actuator (not shown) driven by the pressure fluid from the first hydraulicpump 15 and the combined-flow-driven actuator 20 driven by a combined flow of a portion of the pressure fluid from the first hydraulic pump 15 and the pressure fluid from the second hydraulic pump 18, the load pressure on the combined-flow-driven actuator 20 may become high, resulting in a situation that force is required more than speed as mentioned above. In such a situation, it is not preferred to continue combining the pressure fluid from the first hydraulic pump 15 with that from the second hydraulic pump 18 when the operator wants to increase the speed of the other actuator driven by the hydraulic pressure from the first hydraulic pump 15.
Sufficient force can be assured by the delivery pressure of the second hydraulic pump 18. When such a situation arises, it is, therefore, often preferred from the standpoint of overall work efficiency to stop the combination of the flows despite a decrease in the speed of the combined-flow-driven actuator 20 and hence, to make it possible to supply the pressure fluid from the first hydraulic pump 15 in its entirety to the other actuator such that its speed can be increased.
With the foregoing circumstances of the conventional technique in view, the present invention has as an object the provision of a hydraulic drive system for a work machine, which, when a load pressure on a combined-flow-driven actuator in which flows of pressure fluid from two hydraulic pumps are combined becomes higher than a predetermined pressure, forcedly stops the combination of flows of pressure fluid to permit the driving of the combined-flow-driven actuator with the pressure fluid from one of the hydraulic pumps.
To achieve the above-described object, the present invention provides a hydraulic drive system for a work machine, the hydraulic drive system being provided with an engine, a first and second variable displacement hydraulic pumps drivable by the engine, a first group of center-bypassed directional control valves connected to the first hydraulic pump, a second group of center-bypassed directional control valves connected to the second hydraulic pump and including a flow-combining directional control valve, a flow-combining valve connected to a most downstream directional control valve of the first group of directional control valves via a center bypass passage to supply pressure fluid from the first hydraulic pump, in combination with pressure fluid from the second hydraulic pump, to the flow-combining directional control valve in the second group of directional control valves, a flow-combining circuit communicating the flow-combining valve and a supply port of the flow-combining directional control valve with each other, a combined-flow-driven actuator controlled by the flow-combining directional control valve, and a variable displacement controller for performing overall power control such that a total value of an input torque to the first hydraulic pump and an input torque to the second hydraulic pump does not exceed an output torque of the engine, comprising a canceling valve for canceling the flow combination by the flow-combining valve when a load pressure on the combined-flow-driven actuator becomes higher than a predetermined pressure.
According to the present invention constructed as described above, the flow-combining directional control valve is changed over to actuate the flow-combining valve such that the pressure fluid from the first hydraulic pump is supplied to the supply port of the flow-combining directional control valve via the flow-combining valve and the flow-combining circuit to drive the combined-flow-driven actuator with the combined pressure fluid of the pressure fluid from the first hydraulic pump and that from the second hydraulic pump. An increase in the load pressure on the combined-flow-driven actuator beyond the predetermined pressure in the course of this driving of the combined-flow-driven actuator actuates the canceling valve to cancel the combination of the flows so that the supply of the pressure fluid from the first hydraulic pump to the flow-combining directional control valve via the flow-combining circuit is forcedly stopped. As a result, only the pressure fluid from the second hydraulic pump is supplied to the combined-flow-driven actuator via the flow-combining directional control valve. In other words, the combined-flow-driven actuator is brought into a situation where it is driven only with the pressure fluid from the second hydraulic pump.
Although the input torque to the second hydraulic pump becomes greater as the load pressure on the combined-flow-driven actuator becomes higher, the input torque to the first hydraulic pump which is not affected by the load pressure on the combined-flow-driven actuator can be rendered smaller accordingly. It is, therefore, possible to keep small the total value of the input torques to the first and second hydraulic pumps. As a consequence, it is possible to reduce an increase in the output torque from the engine.
Next assume that combined operation of another actuator, which is driven and controlled by a directional control valve included in the first group of directional control valves connected to the first hydraulic pump, and the combined-flow-driven actuator is being performed. When the load pressure on the combined-flow-driven actuator becomes higher than the predetermined pressure in the situation that the combined-flow-driven actuator is driven by the combined flow of the portion of the pressure fluid from the first hydraulic pump and the pressure fluid from the second hydraulic pump, the combination of the flows is canceled as mentioned above. The pressure fluid from the first hydraulic pump is, therefore, not supplied to the combined-flow-driven actuator, thereby making it possible to supply the pressure fluid from the first hydraulic pump to the above-mentioned other actuator only. Further, the delivery pressure of the first hydraulic pump is no longer affected by the load pressure on the combined-flow-driven actuator, said load pressure having increased beyond the predetermined pressure, and therefore, can be kept smaller compared with the delivery pressure of the second hydraulic pump. This makes it possible to assure a relatively large flow rate in accordance with the so-called P-Q characteristics (pump-delivery pressure characteristics), thereby making it possible to assure sufficient force by the combined-flow-driven actuator and also to increase the speed of the other actuator during such combined operation.
In the above-mentioned construction, the canceling valve may be arranged in a circuit communicating the center bypass passage, which is located between the most downstream directional control valve of the first group of directional control valves and the flow-combining valve, and a reservoir with each other, and may be set to be actuatable responsive to a pressure in the flow-combining circuit.
Further, in the above-mentioned construction, the canceling valve may be incorporated in the flow-combining valve.
In the hydraulic drive system constructed as described above, the flow-combining valve and the canceling valve are formed as an integral unit, thereby achieving a reduction in size.
Furthermore, in the above-mentioned construction, the work machine may be a hydraulic excavator, and an attachment drivable by said combined-flow-driven actuator may be a predetermined accessory mounted on a free end of an arm.